Red algae are able to live at greater depths than green because of the types of light energy that they harvest. Red algae contain chlorophyll "a" and a pigment called phycobilin which enables them to photosynthesize blue light which travels down much farther than typical sunlight. Green algae contain both chlorophyll "a" and "b" which allows them to only photosynthesize sunlight energy. Thus green algae can only grow and survive in the littorals of bodies of water or as far down as sunlight can reach where as red algae will thrive as far down as blue light energy can reach.

Not true. Both red and green algae can use blue light. Chlorophil a can do that. The phicobilin pigments that red algae have enable them to use green and yellow light. so, light usage is like this:
green algae: mostly red and blue
red algae: mostly blue and green, but also some yellow light.

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

other factors in photosynthesis are carbon dioxide levels and temperture, which differ at the levels for red vs green algae.. but I think the main reason is the accessory pigments (phycobilins & phycoerythrin) which absorb the green light that is mostly present at great depts.

ps: some red algae also have chlorophyll d

"How far you go in life depends on your being tender with the young, compassionate with the aged, sympathetic with the striving and tolerant of the weak and strong. Because someday in life you will have been all of these".

"How far you go in life depends on your being tender with the young, compassionate with the aged, sympathetic with the striving and tolerant of the weak and strong. Because someday in life you will have been all of these".

Look on page 240 and 514 of your textbook : Biology The Dynamics of Life.I had gotten a little confused on that question too, but these pages really helped me to see that the main reason was the red algae have these phycobilins that can absorb green, violet , and blue light. In which can penetrate water below 100 meters deep! Hope I helped a little! Keep studying